Close Look at Young Star Finds a Chemical Surprise

The basics of star formation are easy. Find an unusually dense region within a molecular cloud filled with dust and gas in interstellar space and let gravity do the rest. The gas and dust will eventually coalesce into a doughnut-shaped envelope that encircles an inner rotating disk. As material accumulates over hundreds of thousands of years, the central region collapses into a star while the disk solidifies into planets.

Astronomers have understood this overall scenario for decades, but the details are fuzzy because telescopes haven’t been good enough to check theorists’ computer models. That changed in 2011 with the partial completion of the Atacama Large Millimeter/submillimeter Array (ALMA). The collection of radio antennas is being erected on the Chajnantor Plain, 5000 meters above sea level in the Chilean Andes, where the dry, sparse air causes minimal distortion of the faint waves from the far reaches of the universe. Using 24 of the antennas—the final array will have 66—an international group led by astrophysicists at the University of Tokyo, has taken the most detailed look yet at the heart of a star-forming region and found a chemical surprise.

The researchers trained ALMA on a very young star still forming in the constellation Taurus, about 450 light-years from Earth. As is typical at such an early stage, the star is encircled by an envelope and disk of gas and dust. The new scope’s power enabled the team to identify the chemical composition of the gases at different locations throughout this star- and planet-forming system. Previously, astronomers thought that the envelope and disk must be made up of the same gaseous molecules of hydrogen found throughout interstellar space plus dust particles made up of other elements. To the surprise of the University of Tokyo group, ALMA detected something different—sulfur monoxide gas—in a narrow band where the envelope meets the disk. Collisions between particles in the envelope and those in the rapidly spinning disk generate heat that thaws frozen sulfur monoxide molecules stuck to dust grains, explains Nami Sakai, an astrophysicist at the University of Tokyo. Sulfur monoxide can’t be detected when it is frozen to dust grains. But ALMA can spot it in its gaseous state. Knowing just what gases are swirling around young stars should lead to a better understanding of where and how elements found in planets, comets, and asteroids are formed. Sakai and colleagues report their findings online today at Nature.

"These are beautiful data and very interesting results," says Ewine van Dishoeck, an astrophysicist at the Leiden Observatory in the Netherlands. "This work shows that ALMA will provide ample observational evidence" that will challenge theoretical models, adds astrophysicist Stéphane Guilloteau of University of Bordeaux in France. "This paper is a beautiful example of the new discovery [capabilities] offered by ALMA."